Intraoperative Neurophysiological Monitoring with Motor Evoked Potentials during MCA Aneurysm Surgery

Introduction: Motor evoked potentials (MEPs) were evaluated during MCA aneurysm surgery. Intraoperative changes of MEPs are correlated to postoperative motor status and neuroradiologial findings. Methods: 70 patients (38 at facility 2, 32 at facility 1; 14 m, 56 f; mean age 54 years) undergoing MCA aneurysm surgery were studied. MEPs were elicited by transcranial electric or direct cortical stimulation with a multipulse anodal stimulation (train of 5 stimuli; single pulse duration 0.5 ms, interstimulus interval 2–4ms) and recorded from bilateral thenar, anterior tibialis and contralateral biceps and finger extensor muscles. Permanent or transient loss of MEPs was considered as significant. Motor status was evaluated pre- and immediately postoperatively and at time of hospital discharge. Neuroradiologic lesions were evaluated by comparing pre- and postoperative brain CT scans. Results: In all patients, MEPs were present at the beginning of surgery. In 9 (13%) patients significant MEP changes occurred. 4/9 patients (44%) with permanent MEP losses had a severe motor deficit without significant improvement in 3 of those 4 patients. In all those 4 patients, brain CT scans demonstrated ischemic lesions within the motor cortex or subcortically. 1/5 patients with transient MEP losses developed a moderate hemiparesis without complete recovery. The remaining 4 patients did not have any motor deficit. 3 patients without intraoperative MEP changes developed a transient slight motor deficit recovering within a week. None of those patients had an ischemic lesion within the primary motor cortex or its pathways. Conclusions: 1) Intraoperatively preserved MEPs always correlate with good motor outcome. A mild postoperative motor deficit – despite preserved MEPs – will recover within the first postoperative week. 2) Transient MEP loss might be followed by a motor deficit, which is slight or transient. 3) The intraoperative permanent loss of MEPs is always followed by a severe motor deficit and neuroradiologic lesions within the motor cortex and motor pathways.